1. Field of the Invention
The present invention relates to a liquid crystal projector for projecting a full-color image on a projection surface, such as a screen, and, more particularly, to an apparatus which uses three liquid crystal display elements to display images of three primary colors, red, green and blue, respectively, combines the red, green and blue image lights from those liquid crystal display elements into one full-color image light and projects this light.
2. Description of the Related Art
FIG. 1 illustrates the structure of a liquid crystal projector of this type.
This liquid crystal projector comprises three liquid crystal display elements 1R, 1G and 1B, a light source 2, two color-separating dichroic mirrors DM1 and DM2 for separating the light from the light source 2 into lights R, G and B of the three primary colors (red, green and blue), two color-combining dichroic mirrors DM11 and DM12, which constitute an optical system for combining image lights emanating from the three image display elements 1R, 1G and 1B, and a projection lens 3. Those components are arranged as illustrated in FIG. 1.
The liquid crystal display element 1R displays an image with a red component and will therefore be referred to as "red image display element." The liquid crystal display element 1G displays an image with a green component and will therefore be referred to as "green image display element." The liquid crystal display element 1B displays an image with a blue component and will therefore be referred to as "blue image display element."
The individual image display elements 1R, 1G and 1B are matrix liquid crystal display elements of the same structure. The red image display element 1R is driven for its display action in accordance with the red-component image data among image data with individual color components of red, green and blue that offer a full-color image. The green image display element 1G is driven for its display action in accordance with the green-component image data, and the blue image display element 1B is driven for its display action in accordance with the blue-component image data.
The light source 2 comprises a high-luminance lamp 2a for emanating white light, and a reflector 2b for reflecting the light from the lamp 2a toward the first color-separating dichroic mirror DM1. The reflector 2b is a parabolic reflector to reflect the light from the lamp 2a in the direction parallel to the reflector's optical axis.
The first color-separating dichroic mirror DM1 has a characteristic to reflect light of a green waveband among the white light from the light source 2 while passing light of the other wavebands. The green light G separated by this dichroic mirror DM1 is reflected at a mirror M1 to enter the green image display element 1G.
The second color-separating dichroic mirror DM2 has a characteristic to reflect light of a red waveband among the light RB which has passed first color-separating dichroic mirror DM1 and includes light of a red waveband and light of a blue waveband, and to pass the light of the blue waveband. The red light R separated by this dichroic mirror DM2 enters the red image display element 1R, while the blue light B enters the blue image display element 1B.
Color purity compensation filters FR, FG and FB are respectively arranged on the light-incident sides of the individual display elements 1R, 1G and 1B, so that the color purities of the red, green and blue lights R, G and B separated by the color-separating dichroic mirrors DM1 and DM2 are compensated for by those filters FR, FG and FB, respectively, before entering the respective display elements 1R, 1G and 1B.
FIG. 2 illustrates wavebands of the red, green and blue lights R, G and B separated by the color-separating dichroic mirrors DM1 and DM2; the wavebands of the red light R and the green light G lap over each other around 600 nm, and the wavebands of the green light G and the blue light B lap over each other around 500 nm.
Accordingly, of the red, green and blue lights R, G and B, the green light G of an intermediate waveband includes the red and blue components though slightly, while the red light R and blue light B slightly includes the green component. If the different-color components included in those lights R, G and B are cut off by the color purity compensation filters FR, FG and FB, lights R, G and B with high color purity can enter the respective display elements 1R, 1G and 1B.
The red light R entering the red image display element 1R becomes red image light Ra after passing therethrough, the green light G entering the green image display element 1G becomes green light G after passing therethrough, and the blue light B entering the blue image display element 1B becomes blue image light Ba after passing therethrough.
The red image light Ra leaving the red image display element 1R and the green image light Ga leaving the green image display element 1G enter the first color-combining dichroic mirror DM11, while the blue image light Ba leaving the blue image display element 1B is reflected at a mirror M2 to enter the second color-combining dichroic mirror DM12.
The first color-combining dichroic mirror DM11 has a characteristic to pass light of a green waveband and reflect light of the other wavebands. The green image light Ga passes through the dichroic mirror DM11 and the red image light Ra is reflected by that mirror DM11, so that both image lights Ga and Ra are combined to yield image light RaGa of a combined color of red and green, i.e., yellow.
The image light RaGa combined by the first color-combining dichroic mirror DM11 enters the second color-combining dichroic mirror DM12 to be combined with the blue image light Ba which also enters this dichroic mirror DM12.
The second color-combining dichroic mirror DM12 has a characteristic to pass light of red and green wavebands and reflect light of the other wavebands. The red-green combined image light RaGa passes through the dichroic mirror DM12 and the blue image light Ba is reflected by that mirror DM12, so that both image lights RaGa and Ba are combined to yield full-color image light RaGaBa having the three primary colors, red, green and blue, combined. This full-color image light RaGaBa enters a projection lens 3, which increases the luminous flux of the received light and projects the resultant image light on a projection surface, such as a screen (not shown).
The liquid crystal projector has luminous flux reduction lenses 4 disposed on the light-leaving sides or light-incident sides of the individual display elements 1R, 1G and 1B; the location is on the light-leaving side in FIG. 1.
Those luminous flux reduction lenses 4 serve to reduce the luminous fluxes of the image lights Ra, Ga and Ba from the display elements 1R, 1G and 1B. Since those luminous flux reduction lenses 4 can reduce the luminous flux of the full-color image light RaGaBa or the combination of the image lights Ra, Ga and Ba before this light RaGaBa enters the projection lens 3, the projection lens 3 can be constituted of relatively inexpensive lenses with smaller diameters, thus contributing the reduction of the manufacturing cost of the liquid crystal projector.
In general, the projection lens 3 is constituted of a combination of multiple high precision lenses, and each precision lens becomes more expensive with an increase in its diameter. A large-diameter projection lens therefore becomes very expensive. If the fullcolor image light RaGaBa is sent to the projection lens 3 without reducing its luminous flux, the projection lens 3 should have a large diameter, resulting in an inevitable increase in the manufacturing cost of the liquid crystal projector.
If the full-color image light RaGaBa is sent to the projection lens 3 after reducing its luminous flux, the projection lens 3 can have a smaller diameter, so that the projection lens 3 can be constituted of relatively inexpensive lenses of smaller diameters, thus ensuring significant reduction of the manufacturing cost of the liquid crystal projector.
As means of inputting full-color image light RaGaBa to the projection lens 3 after reducing its luminous flux, a luminous flux reduction lens may be arranged on the light-incident side of the projection lens 3. In this case, however, the luminous flux reduction lens needs to have a large luminous flux reduction ratio, and one still has to consider the deformation of the projected image due to the lens aberration of that luminous flux reduction lens.
In this respect, the above liquid crystal projector has the luminous flux reduction lens 4 located on the light-leaving side or light-incident side of each of the display elements 1R, 1G and 1B. This arrangement provides a long optical path from the luminous flux reduction lenses 4 to the projection lens 3, allowing each luminous flux reduction lens 4 to have a smaller luminous flux reduction ratio so that the lens aberration need not be concerned with.
As each luminous flux reduction lens 4 can be of an inexpensive type, such as a Fresnel lens, the total cost of the luminous flux reduction lenses 4 if provided for the respective display elements 1R, 1G and 1B becomes less.
In the above liquid crystal projector, the components, such as the display elements and dichroic mirrors, are arranged in such a way that the optical paths from the light source to the individual display elements 1R, 1G and 1B are equal to each other and the optical paths from the individual display elements 1R, 1G and 1B to the projection lens 3 are equal to each other. The luminous flux reduction lenses 4 provided for the respective display elements 1R, 1G and 1B can therefore be of the same type.
The liquid crystal projector having the luminous flux reduction lenses 4 on the light-leaving sides or light-incident sides of the display elements 1R, 1G and 1B still has a problem of producing irregular colors on a full-color image projected on the projection surface.
This is because that the characteristics of the color-combining dichroic mirrors DM11 and DM12 which combine the image lights Ra, Ga and Ba from the respective display elements 1R, 1G and 1B vary in accordance with the incident angle of incident light.
More specifically, a dichroic mirror is designed to show a predetermined spectral characteristic to light incident to the incident surface at an angle of 45.degree., so that it shows a spectral characteristic shifted on the low wavelength side or high wavelength side with respect to light incident at an angle larger or smaller than 45.degree.. It is to be noted that the range of the shift of the spectral characteristic of a dichroic mirror to the shift of incident angle is generally .+-.1 to 3 nm to a shifted angle of 1.degree..
In the liquid crystal projector having the luminous flux reduction lenses 4 on the light-leaving sides or light-incident sides of the display elements 1R, 1G and 1B, since the lights leaving the display elements 1R, 1G and 1B enter the color-combining dichroic mirrors DM11 and DM12 after having their luminous fluxes reduced, the incident angle to the dichroic mirrors DM11 and DM12 is almost 45.degree. in the center portion of each mirror and becomes larger than 45.degree. as the incident point approaches one end from the mirror's center portion while becoming smaller than 45.degree. in the direction toward the other end from the center portion.
Accordingly, the color-combining dichroic mirrors DM11 and DM12 show predetermined spectral characteristics to light incident to their center portions while showing spectral characteristics shifted on the low wavelength side or high wavelength side to light entering on either end.
For instance, the first color-combining dichroic mirror DM11 passes light of every wavelength of the green image light Ga and reflects light of every wavelength of the red image light Ra in the center portion, while cutting off part of either the green image light Ga or red image light Ra on both ends of the mirror.
That is, the spectral characteristic of the dichroic mirror DM11 shifts on the low wavelength side on one end of this mirror and shifts on the high wavelength side on the other end, so that with the spectral characteristic shifted on the low wavelength side, the green image light Ga which is light of a lower waveband than the red image light Ra has its high-wavelength side component cut by the dichroic mirror DM11. With the spectral characteristic shifted on the low wavelength side, likewise, the red image light Ra or light of a high waveband has its low-waveband side component cut by the dichroic mirror DM11. What is cut by the dichroic mirror DM11 is light of a waveband corresponding to the amount of shift of the spectral characteristic of the dichroic mirror.
The image light RaGa, the combined light of the green image light Ga having passed the dichroic mirror DM11 and the red image light Ra reflected by this mirror DM11, has a color with well-balanced red and green components in the center portion of the luminous flux, but shows a poor green component on the side corresponding to one side of the dichroic mirror DM11 while showing a poor red component on the side corresponding to the other side of the dichroic mirror DM11.
This is true of the second color-combining dichroic mirror DM12. Therefore, the full-color image light RaGaBa, the combined light of the image light RaGa having passed the dichroic mirror DM12 and the blue image light Ba reflected by this mirror DM12, has a color with well-balanced red, green and blue components in the center portion of the luminous flux, but shows a poorer green component on the side corresponding to one side of the dichroic mirror DM12 while showing a poor blue component on the side corresponding to the other side of the dichroic mirror DM12. The full-color image to be projected on the projection surface will inevitably have irregular color.
Conventionally, therefore, the transmission wavebands of the color purity compensation filters FR, FG and FB located on the light-incident sides of the respective display elements 1R, 1G and 1B are narrowed to restrict the wavebands of the red, green and blue lights R, G and B to be incident to the display element 1R, 1G and 1B as shown in FIG. 3, thereby narrowing the wavebands of the red, green and blue image lights Ra, Ga and Ba which respectively leave the display elements 1R, 1G and 1B.
With the use of this design, even if the spectral characteristics of the color-combining dichroic mirrors DM11 and DM12 shift in accordance with the shift of the incident angle of light, the red, green and blue image lights Ra, Ga and Ba leaving the respective display elements 1R, 1G and 1B hardly include light of wavelengths which are cut by the dichroic mirrors DM11 and DM12. The light of every wavelength incident on either end of each of the dichroic mirrors DM11 and DM12 will therefore pass or will be reflected by the associated dichroic mirror, improving the color balance of the red, green and blue of the full-color image light RaGaBa combined by the dichroic mirrors DM11 and DM12 to ensure projection of a full-color image without color irregularity.
If the wavebands of the red, green and blue image lights Ra, Ga and Ba leaving the display elements 1R, 1G and 1B are narrowed in the above manner, however, the amount of each image light Ra, Ga or Ba will be reduced accordingly, thus darkening the full-color image to be projected on the projection surface.